Weight training systems such as free weights, weight machines, and resistance bands are useful for increasing strength, but have limitations. The limitations arise from the fact that the amount of weight that can be lifted by a person varies greatly over the range of motion of the involved limb. For example, when performing a bicep curl, the amount of weight-lifting force the bicep can exert is small at the start of exercise, when the arm is fully extended. The amount of weight-lifting force the bicep can exert when lifting a weight (concentric motion) is large when the elbow is bent about 90°, and decreases greatly as the biceps nears complete contraction and the elbow is bent to its fullest extent. Also, the amount of force the bicep can exert when lowering a weight (eccentric motion) is larger than the amount of weight lifting force that the bicep can exert. However, free weights and weight machines apply a single level of resistance over the entire range of motion of the limb, and apply the same level of resistance in both concentric and eccentric motion of the limb. When using a weight machine, a user must limit the amount of weight to be lifted to that weight which can be lifted when the arm is fully extended, and this is lower than the amount of weight the biceps can lift when bent at a 90° angle. The user cannot increase the resistance of the system for eccentric motion. Some weight lifting machines can increase the amount of resistance applied during lifting, but they cannot adjust the amount of resistance applied when the user is lowering a weight. Thus, when using a weight machine or free weights, a user cannot fully recruit and challenge the biceps during a biceps curl. Band resistance systems that rely on large rubber bands suffer the same problem, and also suffer from the additional problem that, as the bands are stretched, the resistance rises, so that the strength of the bands must be limited to that which can be lifted near the end of a curl, when the elbow is bent to its maximum extent (and to a weak position) while the band is stretched to its maximum extent, and the muscles cannot be exercised to the fullest extent possible. This leaves the entire movement up to that point under stimulated. As with weights and weight machines, eccentric phase is also under stimulated.